Geochemical characteristics and origin of oil from different strata in Shunbei oil and gas field, Tarim Basin

Volatile-light oil or oil sands have been found in the Ordovician, Silurian and Cretaceous strata in Shunbei oil and gas field, Tarim Basin. The geochemical characteristics of oil or oil sands from these strata are obviously different. Generally, they can be divided into two genetic types. (1) Oil in the Ordovician and Silurian:The Pr/Ph value is <1, the carbon isotope of the whole oil and n-alkanes is light, and the C₂₁TT/C₂₃TT ratio is also <1. The dibenzothiophene content is dominant among dibenzothiophene, dibenzofuran and fluorene. These features all indicate that the oil is marine sapropelic. But their maturities are different. Oil is mature in the Silurian and highly mature in the Ordovician. (2) Oil in the Cretaceous:The Pr/Ph value is>1.6, the carbon isotope of the whole oil is relatively heavier, and the C₂₁TT/C₂₃TT ratio is>1. The oil is rich in C₂₄ tetracyclic terpenoids and C₃₀ rearranged hopanes. Thus, they are of continental origin. Altogether, oil in the Ordovician and Silurian strata is from the Cambrian source rocks, which are isogenous and non-synchronous reservoirs. Oil in the Cretaceous strata mainly comes from continental source rocks of the northern Kuche Depression.